Mixed metal oxides are used as an electrode active material of a secondary battery. Among secondary batteries, a lithium secondary battery has already been put into practical use as a small-sized power source for, e.g., a mobile phone and a notebook computer; can be used as a power source for vehicles such as an electric vehicle and a hybrid vehicle, and a large-sized power source such as a distributed power source for power storage, and therefore the demand therefor is on the rise. However, in a lithium secondary battery, many rare metal elements such as lithium are used as a raw material for its electrode active material, so that there is a problem of supplying of the above-described raw material in response to increased demand for a large-sized power source.
In response, as a secondary battery that can resolve the supply problem of the battery materials, a sodium secondary battery has been studied. A sodium secondary battery can be composed of materials that are rich in resources and also inexpensive, and it is expected that, by putting it into practical use, a large-sized power source will be supplied in large quantities.
As an electrode active material for a sodium secondary battery, a mixed metal oxide, NaFeO2, is known (JP 2005-317511 A).